Before the advent of the Giori Press, which allowed easy multicolor printing, the United States Post Office issued primarily single colored stamps. This was because bi color stamps, under the traditional engraving process, required different runs through the press for each color. This was time consuming and created "registration" problems, that is problems of the colors aligning correctly one to the other. But other, more disastrous, problems, such as inverts, could occur.
To 1960, the United States issued less than 20 bi colored stamps. The first were the higher values of the 1869 with two of the three values being known inverted. The next issue was 32 years later, the 1901 Pan American issue and two of these values are known inverted. And of course there is the famous 24